My son complained about not having a convenient USB port to charge when we boondock so I added one by tapping into a light next to his bunk in out 282dbhs. I installed them in my TV as well. Simple and inexpensive.
The component you're adding dictates the size of the fuse. If the item pulls 5 amps then fuse it for a 10. If it pulls 8 amps fuse it for a 10. In the case of the USB it probably provides 2.1 @5v. I would fuse it at a 5 amp.. Not because of the voltage only because it pulls so little power.
And also remember if you are pulling power from "somewhere" like a light per the OP, that circuit is already fused or breakered albeit probably at a higher amperage.
Chris & Donna
2014 Voltage Epic 3990, 600AH Lifeblue, 1895W Solar, 3K Magnum w/AGS, 3X Micro-Air ES
2018 RAM 3500 Laramie Sport Crew Cab 6.7 Cummins, AISIN 4X4 DRW 4.10 Long Bed Dually 2004 Sportster 1200c